This invention relates to a continuous direct esterification and polymerization process for producing fiber-forming polyester from ethylene glycol and terephthalic acid. The invention is more particularly concerned with continuous recovery and reuse of ethylene glycol vaporized in the polymerization reaction.
Continuous processes for direct esterification of terephthalic acid with ethylene glycol are disclosed in Leybourne III U.S. Pat. No. 3,590,072 Lewis et al. U.S. Pat. No. 3,676,485, Balint et al. U.S. Pat. No. 3,697,579 and Chapman et al. U.S. Pat. No. 3,927,982. The terephthalic ester is subsequently converted to a high molecular weight polyester by condensation polymerization under low absolute pressure conditions. Higgins U.S. Pat. No. 3,506,622 illustrates a process in which the terephthalate ester passes continuously from the esterification reaction into a prepolymerizer, wherein an initial polymerization occurs at about 250.degree. to 285.degree. C and absolute pressures of about 5 to 60 millimeters mercury, and the prepolymer formed passes into a final polymerizer wherein further polymerization is conducted at similar temperatures but a lower pressure of about 0.5 to 10 millimeters mercury to produce polyester for extrusion into fibers or filaments. The Higgins patent teaches that ethylene glycol recovered from the polymerization can be used directly in the direct esterification reaction without detrimental effects.
In a continuous process of the above type, vapors are formed during polymerization which consist primarily of ethylene glycol but which also contain from about 2 to about 35 weight percent of water vapor. It is important to recover the ethylene glycol for reuse. The vapors from the polymerization vessels can be condensed and recycled for use in the feed to the direct esterification reaction without removing the water. However, a highly efficient process is required for condensing the vapors, since the capacity and the efficiency of vacuum pumping means for maintaining the low pressure used in polymerization depends on the load of uncondensed vapors.